1. Field of the Invention
This invention relates to a recording liquid to be used for various recording tools such as writing implements, printers, etc. (hereinafter called an ink), particularly an ink suitable for the ink jet recording system in which recording is performed by permitting liquid droplets to fly from an orifice of recording head and also to a recording method by use thereof.
2. Description of the Related Art
The ink jet recording system has the advantages of less noise generation during recording as well as easy adaptation to color recording and capability of giving recorded images of high resolution at high speed. In the ink jet recording system, solutions of various water-soluble dyes dissolved in water or mixtures of water with organic solvents have been used as the ink. In this case, the requisite characteristics of the ink to be used are physical values of viscosity, surface tension, etc., within appropriate ranges, high dissolving stability of the dissolved components, no clogging of minute orifices, capability of giving recorded images with sufficiently high density, no change in physical values or no precipitation of solid components during storage, etc. Further, in addition to the above characteristics, it is also required to have the properties such that recording may be performed without restriction with respect to the kind of recording medium, that the fixing speed may be great, that the recorded images may be excellent in light resistance, water resistance, and solvent resistance (particularly alcohol resistance), and also that recorded image with excellent resolution may be given.
In the prior art, as the colorant of the ink in the ink jet recording system, water-soluble dyes have been primarily employed o0 account of the liquid medium being aqueous and, by use of water-soluble dyes, many of the above basic requirements for the ink jet recording system have been satisfied. However, when water-soluble dyes are used, since these water-solubles are inherently inferior in light resistance, problems are frequently involved in light resistance of the recorded images. More specifically, when recorded images are exposed to the light from the light sources such as sun light, fluorescent lamp or projector, the recorded images may be extinguished, or become less readable, or the image may be faded during a prolonged term of storage. Accordingly, investigations have been made regarding the addition of various additives such as UV-absorbers or antioxidants for improvement of light resistance with the basic characteristics maintained as mentioned above for the ink for ink jet recording. Thereby, it has been expected to improve greatly light resistance of the recorded images by addition of such substances into an ink.
However, this kind of additive is inferior in solution stability in the liquid medium of ink particularly in solution stability in water, thus ensuing the problems that it precipitates as solid in the ink with lapse of time, or that it precipitates the dye by decomposing or isomerizing the dye, thereby causing generation of clogging of nozzles.
The ink jet recording method, although it has been said not to be particular as to paper and more easily adapted to plain paper as compared with other recording methods, is not yet satisfactory under the present condition. More specifically, when printing is effected on plain paper commonly used in offices other than the paper for ink jet recording, such as notebooks, reporting paper, copying paper, letter paper, etc., the printed ink will spread along the fibers of the paper, whereby dot shape becomes unstable to generate so called feathering and therefore fine ruled lines, fine letters or complicated chinese characters become frequently unclear and difficult to see.
Further, since the papers as mentioned above are subjected to the treatment called "sizing" so that feathering ma be retarded in letters written by a writing implement with the use of an aqueous ink for fountain pen, the ink cannot easily penetrate into the paper in the printing conducted when by an ink jet printer, resulting in poor drying characteristic at the printed portion, whereby there are involved the problems such that the printed letters are stained by smearing with the cover of the printer and that the ink is transferred to the hand when the printed matter is touched with a hand.
Accordingly, various improvements have been done for the purpose of improving feathering and drying characteristic of printed letters. For example, it has been attempted to make the pH of the ink alkaline or to add a large amount of a surfactant in the ink, etc. An ink having a pH of strongly alkaline is dangerous when the ink is touched with hands and also involves the drawbacks that it is not sometimes good in both feathering and drying characteristic for papers in which a certain kinds of sizing agents are employed. On the other hand, addition of a large amount of a surfactant causes such problems that feathering occurred remarkably depending on the paper, that the ink was not ejected but retracted from the orifice face depending on the conditions of the print head or contrariwise the whole orifice face was wetted to give no ejection of ink, etc.
Although various improvements other than those as described above have been hitherto attempted, under the present condition, no ink has been known which can solve all of the problems of aforesaid feathering and drying characteristic of printed letters.